1. Field of the Invention
The invention pertains to an FCC catalyst comprising zeolite particles which are coated with an inorganic oxide, and to the use thereof in FCC processes.
2. Discussion of Related Art
Hydrocarbons can be catalytically converted in a process in which the hydrocarbon feed is brought into contact with fluidised catalyst particles under appropriate conditions in a reaction zone. The catalyst used in this process is a so-called FCC catalyst which comprises zeolite particles in a matrix. In the process, the catalyst particles are deactivated by the precipitation of coke, which is formed as a by-product of the cracking process, on the catalyst particles. The (partially) deactivated catalyst particles are removed from the reaction zone, freed of volatile components in a stripping zone, subsequently passed to a regeneration zone, and, following their regeneration by combustion of the coke with an oxygen-containing gas, fed back to the reaction zone.
An additional problem encountered in the cracking of heavy hydrocarbon feeds is that these feeds can contain large quantities of contaminant metals, including vanadium and nickel, which are harmful to the zeolite present in the catalyst. It is therefore desirable to protect the zeolite in an FCC catalyst to be used in the cracking of heavy hydrocarbon feeds from the detrimental effect of these contaminant metals.
Japanese laid-open S58-112,051 (laid-open date Jul. 4, 1983) proposes to protect the zeolite particles by coating them with an oxide before they are incorporated into the catalyst. The coating process is carried out as follows. The zeolite particles to be coated are dispersed in an aqueous acidic solution of a salt which corresponds to the oxide to be provided. Then, a basic solution is added, which leads to the in situ formation of the desired oxide, which precipitates on the zeolite particles. The resulting coated zeolite particles are isolated from the suspension and dried at a mild temperature, e.g. at 120.degree. C. The Japanese patent publication warns against calcining of the coated zeolite particles, stating that this would result in an undesirable peeling-off of the coating from the surface of the zeolite.
In principle, various advantages can be given for the use of coated zeolite particles in an FCC catalyst over the use of uncoated zeolite particles.
In the first place, an FCC catalyst comprising coated zeolite particles has, if the coating is of high quality, the advantage that it is less rapidly deactivated by contaminant metals present in heavy feeds than an FCC catalyst comprising uncoated zeolite particles. It is assumed that this is because the nickel and vanadium-containing molecules cannot easily pass the oxide layer. Moreover, zeolite particles coated with a high quality coating are less susceptible to blocking of the zeolite pores by coke formation than uncoated zeolite particles, which is advantageous in the cracking of both heavy and light hydrocarbon feeds.
Another advantage of the use of coated zeolite particles in FCC catalysts instead of zeolite particles without a coating is associated with the preparation of the FCC catalyst. Zeolites can be susceptible to high and low pH-values; if they are contacted with media having a very low or a very high pH, there is a risk of the crystallinity of the zeolite being damaged, or, in extreme cases, of the zeolite being partially dissolved. An oxide coating may protect the zeolite against the extreme pH-values that may be encountered in the preparation of FCC catalysts.
There is another advantage relating to the use of oxide-coated zeolite particles from the point of view of processing technique. Suspensions comprising large amounts, i.e. more than 30 wt. %, of uncoated zeolite particles are unstable under storage conditions: as a result, a compact sediment is obtained which is difficult to redisperse. On the other hand, suspensions containing large amounts of coated zeolite particles are stable under storage conditions; no or hardly any sedimentation takes place, and if it does, the sediment can easily be redispersed.
Thus, there are many advantages associated with the use of coated zeolite particles in FCC. However, the quality of the in situ formed coating provided on the zeolite particles by means of the process of said Japanese patent publication is not good enough to realise these advantages to the highest possible extent. Additionally, the in situ coating procedure described in the Japanese patent publication has some further disadvantages which will be discussed below.
The quality of the alumina coating obtained with the process according to the Japanese patent publication can be studied with electron-microscopic techniques such as STEM/EDX, and SEM. With these techniques it can be seem that a coating is present on the zeolite particles. However, the coating is not complete, and adheres quite loosely to the zeolite particles. Further, it appears from STEM/EDX images that when these alumina coated zeolite particles are incorporated into an FCC catalyst using certain preparation procedures, the coating may disappear from the zeolite particles. Not wishing to be bound by theory, the inventors assume that the coating prepared according to the Japanese patent publication is not resistant to the relatively severe conditions under which FCC catalysts may be prepared.
Moreover, if by careful handling of the coated zeolite particles and mild preparation conditions, one were to succeed in preparing an FCC catalyst comprising zeolite particles coated with alumina formed in situ as described in the Japanese patent publication in which the coating is still present on the zeolite particles, another disadvantage becomes clear. The alumina which precipitates on the zeolite particles in the in situ coating processes described by the Japanese patent publication is a relatively amorphous alumina, which displays a highly a selective cracking activity; it cracks hydrocarbon feeds to form coke and light gases. By incorporating zeolite particles coated with amorphous alumina into an FCC catalyst, an undesirable cracking activity is thus added to the catalyst, so reducing the catalyst's selectivity.
Further, the precipitation process described in the Japanese patent publication takes place at high pH values. This is necessary, because the coating oxides will only precipitate at such high pH values. However, as stated before, zeolites may be susceptible to high pH values, and thus the properties of the zeolite, particularly the crystallinity, may be adversely affected during said coating process. A further drawback to the in situ coating process disclosed in the Japanese patent publication consists in that the protons, or proton precursors such as ammonium ions, in the zeolite can be, and often are, exchanged during the coating process with the cations of which the oxides are to precipitate on the zeolite. This process in uncontrollable, and may lead to undesirable alteration of the zeolite properties.
Additionally, the very small hydr(oxide) particles which are formed during the in situ formation of the oxide may block the pores of the zeolite.